This invention relates generally to the method of current feedback taught in the above-referenced related patents that are directed to wideband direct-coupled transistor amplifiers. These prior art amplifiers offer very wide bandwidth and fast settling time, but lack the ability to provide the same precise input offset voltage, low input bias current, and low drift capabilities available in conventional voltage feedback operational amplifiers. The differential input pair of transistors used in the conventional operational amplifier offers precision performance based upon the ability to closely match two or more transistors of the same type, expecially if monolithic integrated circuit techniques are used. The shortcoming in the prior art wideband direct-coupled transistor amplifiers arises primarily from the unconventional input stage in which an NPN/PNP transistor buffer amplifier having high input impedance and low output impedance replaces the conventional differential transistor pair input stage. Given the current feedback inputs described in connection with these types of amplifiers, achieving close matching of NPN and PNP transistors is difficult.
Three embodiments of the present invention utilize the advantages of precision available with a true differential input while retaining the use of current feedback. The performance of these configurations resembles that of an instrumentation amplifier in that the magnitudes of inverting and non-inverting gains can be made equal and in that good common mode rejection can be obtained. However, this is only one special case, and in general the inverting and non-inverting gains can be simultaneously and independently set. A second non-inverting input is also available for setting output reference voltages or for use in applying a second non-inverting input signal. In addition to the three voltage input terminals, low input impedance inverting and non-inverting input terminals also exist for the direct input of current signals. The gain, offset, and common mode properties of the amplifier are set by the selection of three resistor values. Since the amplifiers use current feedback, the -3 db bandwidth, settling time, and other dynamic performance parameters remain nearly constant and independent of gain settings over a very wide range of gain without the need for any change in compensation. The output is a single-ended voltage. Two of these three embodiments of the present invention are particularly well suited for monolithic integration since no high frequency PNP transistors are required.
In the remaining two embodiments of the present invention, some of the flexibility and advantages inherent in the first three embodiments have been sacrificed in order to return to the more conventional unbalanced signal input configuration of operational amplifiers (two gain setting resistors) using current feedback as described in the above-referenced prior art patents. This is done to retain compatibility with those operational amplifiers which are used on such a wide scale in the industry. These last two embodiments of the present invention are special cases of the first three embodiments in that they retain the attendant advantages of current feedback, low input offset voltage, and the absence of high frequency PNP transistors which make monolithic construction difficult and expensive. The advantage of low input bias current is also available if the circuit is carefully designed and process parameters are tightly controlled.